[libc++] Fix triviality of std::pair for trivially copyable types without an assignment operator (#95444)

Since 83ead2b, std::pair would not be trivially copyable when it holds a
trivially copyable type without an assignment operator. That is because
pair gained an elligible copy-assignment-operator (the const version) in
83ead2b in C++ >= 23.

This means that the trivially copyable property of std::pair for such
types would be inconsistent between C++11/14/17/20 (trivially copyable)
and C++23/26 (not trivially copyable). This patch makes std::pair's
behavior consistent in all Standard modes EXCEPT C++03, which is a
pre-existing condition and we have no way of changing (also, it
shouldn't matter because the std::is_trivially_copyable trait was
introduced in C++11).

While this is not technically an ABI break, in practice we do know that
folks sometimes use a different representation based on whether a type
is trivially copyable. So we're treating 83ead2b as an ABI break and
this patch is fixing said breakage.

This patch also adds tests stolen from #89652 that pin down the ABI of
std::pair with respect to being trivially copyable.

Fixes #95428

GitOrigin-RevId: 02af67c88f393cd6998949cc1bf8075553579a42
6 files changed